The present invention relates to storage facilities for highly chilled liquified gases. More particularly, the present invention relates to offshore terminal and submarine storage facilities for liquified energy gases, including liquified natural gas (LNG).
It has been long known to liquify gases, including natural gas, to reduce volume and thereby facilitate transportation and storage. A significant drawback stemming from the liquification and concomitant concentration of high energy gases is the vastly increased threat to safety and potential for devastation.
A liquid natural gas disaster occurred in the Cleveland, Ohio vicinity in 1944 in which hundreds of people were killed and injured. This disaster effectively terminated the use of liquified natural gas in the United States for the next twenty years.
On the other hand, liquid petroleum gases (LPG) such as propane and butane, have been in widespread uses in both rural and industrial energy applications throughout the United States for many years. Manmade, synthetic gases are also known and used for energy and other useful purposes. As local natural gas supplies dwindle, transported and stored liquid energy gases will become an increasingly significant source of energy throughout the world. These liquified gases are of three basic types: natural (LNG), petroleum (LPG), and synthetic (LSG) including artificially produced domestic energy gases (i.e., methane and ethane) and industrial energy gases (i.e., acetylene and propylene). These liquified gases will be a primary source of heat producing energy for the near range future, certainly during the interim until solar, geothermal and fusion energy sources are made practical and economical. Also, of the remaining, readily available energy sources (i.e, coal, oil, gasoline, uranium and gas), only the liquified energy gases burn cleanly, which renders them attractive alternatives as soceity becomes increasingly concerned with the prevention of air pollution.
Natural gas is a mixture of hydrocarbons, typically 65 to 99 percent methane, with smaller amounts of ethane, propane and butane. When natural gas is chilled to below minus 263 degrees Fahrenheit, it becomes an odorless, colorless liquid having a volume which is less than one six hundredth (1/600) of its volume at ambient atmospheric surface temperature and pressure. When LNG is warmed above its -263.degree. F. boiling point, it boils (i.e., regassifies) and expands to its over six hundred times greater original volume. Thus, it will be appreciated that a 150,000 cubic meter LNG tanker ship is capable of carrying the equivalent of 3.2 billion cubic feet of natural gas.
Of the known liquid energy gases, liquid natural gas is the most difficult to handle because it is so intensely cold. Complex handling, shipping and storage apparatus and procedures are required to prevent unwanted thermal rise in the LNG and resultant regassification. Storage vessels, whether part of LNG tanker ships or land-based, are closely analogous to giant thermos bottles with outer walls, inner walls and effective types and amounts of insulation in between.
LNG storage tanks in the United States have heretofor been built mostly aboveground with some frozen pit facilities properly characterized as mostly above ground. Most such tanks have been enclosed by surrounding earthen dikes. Such dikes were sized and emplaced to enclose an area and volume at least as great as the storage capacity of the largest tank within the diked area. Besides the known potential hazards of explosion and inferno created by massive rupture of such tanks, a small rupture, as by a saboteur's bullet or projectile in the upper part of the sidewall could result in a stream of LNG shooting beyond the dike, thereby rendering it useless to contain the hazard of a spill and creating the consequent likelihood of explosion and fiery inferno.
The hazards presented by liquid energy gas storage facilities to adjacent population concentrations are so great that the Controller General of the United States has recommended to Congress that all "future facilities for storing large quantities of these gases should be built in remote areas." This recommendation, as well as others, appeared on the front cover of Volume 1 of a comprehensive three volume Controller General Report to the Congress entitled Liquified Energy Gases Safety, document No. EMD-78-28, issued July 31, 1978. Great public controversy has erupted about the proposed establishment of additional onshore LNG storage facilities, regardless of actual remoteness to populated areas.
Surprisingly, little public attention has heretofore focused upon the ocean and its vastness as a potentially safer environment for storage facilities for liquified energy gases, including LNG. A partially submerged offshore storage tank for liquified energy gases was disclosed in the Jackson U.S. Pat. No. 3,675,431, issued July 11, 1972. That patent described an insulated tank which was prefabricated, floated to a suitable offshore site and then sunk until its submerged base rested on the floor of the sea. An upper above-the-water domed metal cylinder extended from the concrete base. Insulation lined the interior of the tank. A thin and flexible membrane inside the insulation provided the required liquid tight interior lining of the tank. The insulation lining the submerged portion of the tank was said to be thinned, so that a layer of ice formed around the outside of the concrete base when the tank was filled with liquified gas. In accordance with the invention claimed in the patent, the ice layer supposedly acted as an outer seal for the submerged concrete.
Subterranean storage vessels for LNG have been used in Japan with some claimed advantages over surface, landbased storage facilities. Nevertheless, the hazards presented by such facilities, particularly from earthquake damage, remain unabated. Also, inspecting and maintaining such facilities was extremely difficult and hazardous.
Another prior proposal for offshore underwater storage of crude petroleum product was described in the Pogonowski U.S. Pat. No. 3,643,447, issued Feb. 22, 1972. Therein, a frame anchored to the sea floor supported an expansible, bladder-like tank held to the frame at a predetermined depth below the surface. Crude petroleum from an undersea well was piped into the tank continuously and caused it to expand. A delivery conduit from the tank extended to the surface and delivered the crude into tanks of an awaiting barge or ship. Latent hydrostatic lifting pressure developed by sea pressure against the flexible tank was used to force the crude out of the bladder-like tank, through the conduit, and into the awaiting tanker without pumping being required. While the Pogonowski contrivance might have been feasible for storage of liquid crude at ambient sea temperature, there is no suggestion in that reference of the suitability of its subject matter for storage of liquified energy gases and other liquids at low temperatures, the use of ambient water pressure to maintain the liquid state of the liquified gas, or the use of depth in the water to dissipate small leaks from the facility without the danger of fire or explosion.